This invention relates to apparatus for pulverizing, i.e., grinding, material, and more specifically to a vane wheel assembly that is particularly suited for embodiment in a mill wherein the vane wheel assembly is operative to effect a primary classification of the pulverized material.
It has long been known in the prior art to provide apparatus employable for purposes of effecting the grinding of materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used heretofore to effect the grinding of a multiplicity of different kinds of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid apparatus. The existence of such differences is, in turn, attributable for the most part to the diverse functional requirements that are associated with the individual applications in which such apparatus are designed to be employed. For instance, in the selection of the particular type of apparatus that is to be utilized for a specific application, one of the principal factors to which consideration must be given is that of the nature of the material that is to be ground in the apparatus. Coal is one such material wherein there is a need to grind it in order to render it suitable for use in certain applications. Furthermore, fossil fuel fired power generation systems represent one such application in which it is desired to employ coal, as the source of fuel therefor, and wherein a requirement exists to grind, i.e., pulverize, the coal to render it suitable for use for this purpose, i.e., for use in a coal-fired power generation system.
For purposes of the discussion that follows, the coal-fired power generation systems referred to above are considered to consist of essentially the following major operating components: a coal feeder, apparatus for pulverizing coal, a distribution system for distributing the coal after the pulverization thereof, a furnace in which the coal is to be burned, and the requisite controls for effecting the proper operation of the coal-fired power generation system. Of particular interest herein is that portion of the coal-fired power generation system which has been identified above as the apparatus for pulverizing the coal. Coal pulverizing apparatus are not new. They have been known to exist in the prior art for more than half a century. Furthermore, many improvements in the construction and/or mode of operation of coal pulverizing apparatus have been made during this period.
There are a number of features that it is advantageous for any coal pulverizing apparatus to possess, but particularly those which are designed for employment in a coal-fired power generation system. Reference is had here to features such as reliability, low power consumption, minimum maintenance and wide range of capacity. In addition, such apparatus advantageously should also be characterized by quiet operation, integrated lubrication systems, convenient adjustment and control of coal flow and fineness, and the ability to handle the high temperature air that is required for high moisture coal.
One particular type of coal pulverizing apparatus which is to be found in the prior art that is advantageously characterized by the embodiment therein of the above-recited features is an apparatus most commonly referred to in the industry by the name bowl mill. The latter apparatus obtains its name by virtue of the fact that the pulverization, i.e., grinding, of the coal which takes place therein is effected on a grinding surface that in configuration bears a resemblance to a bowl.
Reference may be had, by way of exemplification, to U.S. Pat. No. 2,079,155, the latter being assigned to the same assignee as the instant application, for a teaching of the nature of the construction and the mode of operation of a prior art form of bowl mill that is suitable for use in a coal-fired power generation system to effectuate the pulverization of the coal that is to be burned as fuel therein. As taught by the aforementioned patent, a bowl mill essentially consists of a body portion in which a grinding table is mounted for rotation, a plurality of grinding rollers that coact with the grinding table to effect the grinding of coal interposed therebetween, coal supply means for feeding to the interior of the bowl mill the coal that is to be pulverized, and air supply means for supplying to the interior of the bowl mill the air required in the operation of the latter. In accordance with the mode of operation of such a bowl mill, the coal which enters the bowl mill is pulverized by virtue of the coaction of the grinding rollers with the grinding table. After being pulverized, the coal particles are thrown outwardly by centrifugal forces whereby the particles are fed into a stream of air that is entering the bowl mill. The stream of air, which now contains pulverized coal particles, flows through a tortuous path that is established in part by the positioning within the bowl mill of a suitably supported deflector means. As the stream of air and coal particles flows along the aforementioned tortuous path, the sharp turns contained therein effects the separation of the coarse coal particles from the air stream. These coarse coal particles are then suitably returned to the grinding table for further pulverization, while the fine coal particles are carried through the bowl mill in the air stream, and exit therefrom along with the air. In a conventional coal-fired power generation system, a multiplicity of bowl mills of the type shown in the aforementioned patent would commonly be employed for purposes of satisfying the requirements of the system for pulverized coal.
Although bowl mills constructed in accordance with the teachings of the prior art have, under actual operating conditions, proven capable of providing adequate performance to date, a need has nevertheless been evidenced for improvements to be made therein. One area, for example, in which prior art forms of bowl mills have heretofore been known to be disadvantageously characterized is that of the manner in which the air is made to flow through the deflector portion of the bowl mill. More specifically, reference is had here to the fact that prior art forms of mills have heretofore been disadvantageously characterized by virtue of the fact that the design thereof is such that in accordance therewith the air required in the operation of the mill is made to flow through the deflector portion of the mill at a very high velocity, relatively speaking. On the other hand, it is desirable from an operational and performance standpoint to have the air required in the operation of the mill flow through the deflector portion of the mill at the lowest possible velocity. There are a number of reasons for this. By way of exemplification and not limitation, one of these resides in the fact that a lower velocity results in a lower pressure drop across the bowl portion of the mill. This in turn reduces the total pressure drop across the mill with the concomitant result that for the same flow through the mill of material to be pulverized, it is possible to realize either a gain in air flow or a reduction in power consumption. The significance of a gain in air flow is that greater air flow translates into larger mill capacity or greater drying capability. Another reason is that the lower the velocity of the air in which the particles of pulverized material are entrained, the better the particle classification which occurs within the mill and thus the better the fineness control that one is able to achieve with the mill. A third reason is that the lower the velocity at which the air in which the particles of pulverized material are entrained flows through the deflector portion of the mill, the less the wear on the separator body internals of the mill. A fourth reason is that a lower air flow means that it is possible to reject more pyrites thereby reducing the wear on the upper millside area of the mill. In summary, therefore, a need has been evidenced in the prior art for a new and improved vane wheel assembly, which when employed in a bowl mill so as to be operative to effect a primary classification of the material such as coal that is subjected to pulverization within the mill, is characterized by the low velocity at which the air is made to flow when passing through the deflector portion of the bowl mill.
It is, therefore, an object of the present invention to provide a new and improved vane wheel assembly that is suitably constructed so as to be employable in a bowl mill for purposes of effecting a primary classification of the material that is pulverized in the bowl mill.
It is another object of the present invention to provide such a vane wheel assembly for bowl mills wherein by virtue of the low velocity at which the air is made to flow through the vane wheel assembly there results a lower pressure drop across the bowl portion of the bowl mill and thereby a reduction in the total pressure drop across the bowl mill with the concomitant result that for the same flow of pulverized material through the bowl mill it is possible to realize either a gain in air flow or a reduction in power consumption.
It is still another object of the present invention to provide such a vane wheel assembly for bowl mills wherein by virtue of the low velocity at which the air is made to flow through the vane wheel assembly there results a better classification within the bowl mill of the particles of pulverized material and thus better fineness control.
A further object of the present invention is to provide such a vane wheel assembly for bowl mills wherein by virtue of the low velocity at which the air is made to flow through the vanes wheel assembly the separator body internals of the bowl mill are subjected to less wear.
A still further object of the present invention is to provide such a vane wheel assembly for bowl mills wherein by virtue of the low velocity at which the air is made to flow through the vane wheel assembly the upper millside area of the bowl mill is subjected to less wear.
Yet another object of the present invention is to provide such a vane wheel assembly for bowl mills which is suitable for employment in retrofit applications as well as being suitable for employment in newly constructed bowl mills.
Yet still another object of the present invention is to provide such a vane wheel assembly for bowl mills which is advantageously characterized by its ease of manufacture and its ease of installation in a bowl mill, as well as by the fact that it is relatively inexpensive to provide.